How to generate IPv6 RA without any prefixes?

[sthaug at nethelp.no]

> > In IPv6 it should be possible to generate a Router Advertisement which
> > contains no prefix options (the idea being that I want the host to
> > populate its default router list but nothing else). However, I cannot
> > seem to get rtadvd to do this.
> >
> > If I start rtadvd with no /etc/rtadvd.conf file, it sends RAs with a
> > prefix option corresponding to the IPv6 address of the interface. In
> > the /etc/rtadvd.conf I can explicitly specify prefixes ("addr"), but
> > I can't find any way to specify that no prefix options should be sent.
> >
> > Any suggestions?
> 
> What if you start rtadvd with -s and have a basically empty (default)
> config file entry?
> 
> I haven't checked but theat would be my best guess?

Does not appear to work. With /etc/radvd.conf containing only:

vlan0:\
        :raflags#128:

and starting rtadvd with -s, it still announces the /64 prefix I have
on the vlan0 interface.

It's possible we simply have some missing functionality here. The man
page for Mac OS X shows an "addrs" field which FreeBSD doesn't have -
but which still doesn't do quite what I want:

http://developer.apple.com/library/mac/#documentation/Darwin/Reference/ManPages/man5/rtadvd.conf.5.html

addrs   (num) Number of prefixes.  Its default is 0, so it must explicitly
         be set to positive values if you want to specify any prefix
         information option.  If its value is 0, rtadvd(8) looks up the
         system routing table and advertise the prefixes corresponding to
         interface routes on the interface.  If its value is more than 1,
         you must specify the index of the prefix for each item below.
         Indices vary from 0 to N-1, where N is the value of addrs.  Each
         index shall follow the name of each item, e.g., ``prefixlen2''.

Steinar Haug, Nethelp consulting, sthaug at nethelp.no